I can see the truth in your statement, and I believe that you can read this excellent short war-time story in a way to support it, however, I personally believe that the story doesn't show a gradual change in the protagonist. It rather shows the way in which in situations of conflict or war we have to let a different side of ourselves dominate. I believe that the protagonist is always a sensitive human being, but part of the impact of this story concerns the way in which during war-time, to be successful and survive, we need to squash such sensitive parts of our character and let more violent and ruthless parts dominate.
Note the way that the protagonist is described as the story opens and we are plunged straight into a situation of urban warfare:
On a rooftop near O'Connell Bridge, a Republican sniper lay watching. Beside him lay his rifle and over his shoulders was slung a pair of field glasses. His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of teh fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.
Thus we see the protagonist straight away in his warfare mode. He is engaged in a life-or-death struggle and there is no place for sensitivity on the battlefield of the rooftops where he is. Of course, this continues until straight after his success against his enemy when he is able to kill him. Then, as he watches his opponent fall to the ground, the sniper experiences a sudden change of heart:
The sniper looked at his enemy falling and he shuddered. The lust of battle died in him. He became bitten by remorse. The sweat stood out in beads on his forehead. Weakened by his wound and the long summer day of fasting and watching on the roof, he revolted from the sight of the shattered mass of his dead enemy. His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.
Now, note the sudden change in his character. This is definitely not a "gradual change" as suggested in your statement. This is an immediate response to the end of an incredibly stressful situation when we see the normal character of the protagonist reimpose itself and, in this moment of internal conflict, we see the sensitivity of the protagonist. This is why I believe that in this story we see the way that in warfare we need to supress our true characters and become ruthless warriors.
Day night noon. :D